­Single-car drivers commuting in fossil-fuel burning cars, smog­, pollution, crime -- what other ur­ban scourges can you think of? Half of the world's population currently live­s in urban areas; yet these urban areas make up only 2 percent of the world's land and spend three-quarters of the world's resources [source: MIT]. That's a lot of people in a very small space consuming a great deal.

Between now and the year 2050, urban growth will only continue to rise: 89 million homes and 190 billion square feet (about 17.5 billion square meters) of retail and other nonresidential space will be built in the United States alone [source: National Resources Defense Council]. And in conjunction with that density, pollution is soaring. London, for instance, released about 45 million tons (about 41 million metric tons) of carbon dioxide, a greenhouse gas, into the atmosphere last year [source: IEEE]. By greening cities and neighborhoods around the world, we have the opportunity to make a positive impac­t on global warming.

How do green cities help in the effort against climate change? Eco-cities all share similar characteristics: They aim to reduce or eliminate fossil-fuel use, adopt sustainable building practices, promote "green space" and clean air quality, implement energy-efficient and widely available public transportation, create walkable city designs and develop well-organized mixed-use neighborhoods that combine living, working and shopping. These qualities add up to sustainable urbanism.

Here we'll look at five green cities of the future -- some that have broken ground for construction, some that are still the ambitious aspirations of city planners -- all competing to be the first carbon-neutral city in the world.

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5

San Francisco's Treasure Island, Calif.

Treasure Island's transformation to green is just another shade in its colorful history. The 400-acre (1.5-square-km) man-made island was built in the middle of San Francisco Bay for the 1939 Golden Gate International Exposition. The ­land was intended to hold an airport after the Expo. Plans for the commercial airport never took off, though, after the Navy acquired the island during World War II. In 1996, the base was decommissioned, and since then, the island has been home to a deteriorating landscape and about 3,000 residents.

­Under a new proposal, Treasure Island -- along with its neighbor Yerba Buena Island -- will become one of the most environmentally friendly developments in the United States when ground is broken in 2009.

The Treasure Island project will be a test bed for eco-friendly urban ideas. Some of the proposed green features include LEED-certified buildings, the reduction (or elimination) of storm water runoff, alternative forms of water treatment including artificial wetlands called Living Machines and a transit system that favors clean air vehicles over fossil-fuel chugging cars. A 20-acre (0.08 square-km) city-operated organic farm is planned in walking distance from the city center and will supply the projected 13,500 residents with locally produced foods. Solar and wind-farm power will provide energy; and by 2020, solar panels will cover 70 percent of rooftop space and will provide about 30 million kilowatt hours of electricity annually [source: ENN].

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Greensburg, Kan.

When a tornado blew through the tiny town of Greensburg, Kan. on May 4, 2007, nearly everything was lost including homes, businesses and trees. The roughly 1,500 residents are rebuilding Greensburg as one of the greenest towns in America, and the project has caught the attention of energy producers, among others. Discovery Communications and actor Leonardo DiCaprio are documenting the story of Greensburg's rise from the ashes in a TV series called "Eco-Town" on Planet Green.

4

Victoria, British Columbia

Dockside Green is slated to be North America's first carbon-neutral community.

­How will Dockside Green achieve its goal to be the first carbon-neutral community in North America? Through a combination of green solutions for buildings, transportation, energy and waste treatment.

Let's begin with buildings: Those of Dockside Green are being constructed with reclaimed wood from forests that were submerged by reservoirs. Energy-efficient appliances and fixtures (such as motion-sensing light switches), green roofs (rooftop gardens), and carbon footprint monitors (that allow residents to track their heat, energy and water use over time) are outfitted inside homes.

It's unlikely you'll find a car or two parked in driveways, either. Residents of Victoria, and now Dockside Green, take part in a clean-fuel and hybrid car-sharing program (even the cars are Smart). In addition, Dockside Green plans include bike and pedestrian paths, subsidized public transit and a harbor ferry.

Energy and waste treatment will be self-contained within Dockside Green. One hundred percent of waste will be treated on-site, and the treated water will be reused to flush toilets and irrigate gardens. A biomass-gasification plant will turn wood waste into energy for heat and hot water.

This innovative green community is under way currently, with the first of three neighborhoods opening in 2009. Upon completion, the entire community will be home to about 2,500 people. ­

3

Sherford, England

The Prince of Wales has plans for a cutting edge green city with traditional styling.

Anwar Hussein Collection/ROTA/WireImage

A b­owling green, a dedicated cricket playing field and an organic farm in the town's park -- sounds like a lovely, green place to live, yes? The future residents of Sherford, England, have these amenities to look forward to, along with a plethora of other green characteristics.

­Sherford, in south Devon, is the eco-project of Prince Charles. It will be home to 12,000 people and is planned for completion by 2020. Royal advisors consider it Britain's greenest future community.

The proposed community will take advantage of cutting-edge green building designs and materials but will look like a traditional English town. Buildings will be constructed with sustainable materials gathered mostly from within a 50-mile (80-kilometer) radius of the site; water and sewer waste will be recycled.

Homes and workplaces alike will put their rooftops to work. The majority of buildings will have solar power systems, and vegetation will cover the roofs of commercial buildings. About half of Sherford's power will be supplied from renewable sources in the community: In addition to solar power, plans call for wind turbines.

Lastly, a walkable urban layout will put residences, retail stores and industry in close proximity, reducing the need for cars. In fact, cars will be banned from some areas of the town. Did we mention new homeowners receive a free bicycle? ­

2

Dongtan, China

China is on its way to being the world's worst offender in greenhouse gas emissions by 2009 [source: USA Today]. Yet there's an eco-oasis in the works on Chongming Island, near Shanghai. Chongming Island, roughly three-quarters the size of Manhattan, sits at the mouth of the Yangtze River and is poised to become home to one of the first carbon-neutral cities in the world: ­Dongtan.

­Dongtan will grow in phases, with phase one (for between 10,000 and 25,000 residents) expected to be complete in time for the 2010 Shanghai Expo and the final phase complete by 2030 (for up to 500,000 residents). The city will be comprised of three neighborhoods called Marina, Lake and Pond, and will be designed with opportunities for all socioeconomic groups and many occupations in mind, including farming. Designated agricultural areas will use organic, sustainable farming methods.

It's the energy plan that makes Dongtan ambitiously green. The city will source its energy from a combination of solar and wind power, biofuels and recycled organic matter. There are no plans for landfills in Dongtan -- waste will be composted, processed and reused. Rooftops will be green with vegetation, which provides insulation. Only clean-fuel cars will be allowed on the island, and the abundant public transportation will run on hydrogen fuel cells. In addition, bike and pedestrian paths will snake through the city grid.

The city will only be built on three-quarters of Chongming Island. The remaining area is reserved as a protected ecological zone. To attain carbon-neutrality, Dongtan planners also expect to plant trees to offset emissions. ­

1

Masdar City, Abu Dhabi, United Arab Emirates

No cars, no waste, no pollution. Doable? Such a city is slated to rise from the oil-rich grounds in Abu Dhabi. Masdar, which means "the source" in Arabic, is a $22-b­illion undertaking that could be the world's first carbon-neutral city [source: Masdar].

Masdar's sustainable urban development will take advantage of wind, hydrogen and solar-photovoltaic energy sources. Wastewater will be treated and recycled into irrigation systems.

­In addition, Masdar's transportation goals are ambitious. Fossil-fuel burning cars are banned from the city in lieu of an electric personal light-rail system -- small, programmable cars that run only when you need to go somewhere, and a pedestrian-friendly city layout.

Masdar is already under construction and will develop over several phases, with completion expected in 2016 [source Inhabitat]. Up to 50,000 people are expected to live in Masdar, and the first residents will likely move in sometime during 2009 [CNET].

The time of the carbon-neutral city is upon us. And if these projects make the jump from paper to reality successfully, they might even earn a spot on our list of existing amazing green cities. ­

Green Mountain Resort, Libya

Cyrene, Libya, an area about the size of Wales, may be one of the next carbon-neutral developments in the world. The Green Mountain Sustainable Development Area is a new eco-friendly project in a remote coastal region of the country. The resort, intended to boost tourism, would draw on renewable energy sources, specifically wind and solar power, sustainable transportation and housing, as well as closed-loop water systems.